The disclosure herein relates to a lower leg prosthesis for an amputee who has at least one leg having a lower or upper leg stump. The lower leg prosthesis has a distal end, the distal end forming an artificial foot being connectable to an artificial foot. The lower leg prosthesis also has a proximal end, the proximal end forming a stump socket for connection to said leg stump, or the proximal end being connectable, optionally via an artificial knee, to a stump socket for connection to said leg stump. A use condition of the prosthesis is defined by the distal end actually being connected, if applicable, to said artificial foot, the proximal end actually being connected, if applicable, to said stump socket, and said stump socket actually being connected to said leg stump of said amputee.
Known prostheses for upper or lower leg amputees suffer from a number of drawbacks. Important drawbacks of the known prostheses are lack of balance and relatively high energy consumption during standing and walking During walking with a known prosthesis this is for example visible from the typical gait pattern, showing compensatory trunk movements by the amputee as well as a wider gait to solve balance problems in the frontal plane of the amputee. The frontal plane is the sideways plane, as distinct from the sagittal plane (fore-aft) of the amputee. Many improvements have been made on the design of prosthetic knees and feet, but only little attention has been given on the part between knee and foot, the lower leg, in order to improve lateral balance.
EP1340478A2 discloses a lower leg prosthesis of the type as initially identified hereinabove. This known prosthesis 1, shown in FIGS. 1 through 7B of EP1340478A2, includes a foot portion 3, a leg mounting portion 4 and a parallel linkage 10 connecting these portions. The leg mounting portion 4 has a flat mounting plate 4a. The parallel linkage 10 includes one fixed link 11 and four expansible links 13. The fixed link 11 has an upper end thereof fixed to the mounting plate 4a and a lower end thereof connected to the foot portion 3 via a ball joint 12. Each of the four expansible links 13 has an upper end thereof connected to the mounting plate 4a via an upper ball joint 14a and a lower end thereof connected to the foot portion 3 via a lower ball joint 14b. 
Due to these ball joints 12, 14a and 14b and the expansibility/compressibility of the links 13, the angle of the fixed link 11 with respect to the foot portion 3 can be changed in any desired direction. In short, there are at least three degrees of freedom for the fixed link 11 and, correspondingly, for the mounting plate 4a. Hence, due to this high degree of freedom, an amputee wearing the known prosthesis 1 on his or her living leg stump can tilt the prosthetic lower leg in any desired direction relative to a floor while keeping the artificial foot of the prosthesis 1 resting on the floor and without rotating the artificial foot relative to the floor on which it rests. Various tilting positions of the prosthetic lower leg are shown in FIGS. 5A, 5B, 6A, 6B, 7A and 7B of EP1340478A2. In all these positions the shown artificial foot 6 has not rotated relative to the floor on which it rests due to said high degree of freedom, which in fact is more or less similar to the high degree of freedom that occurs in case of a living leg having a living ankle joint and a living foot.
Due to said high degree of freedom, also this prosthesis 1 known from EP1340478A2 suffers from the abovementioned lateral balance problems, amongst others resulting into the abovementioned compensatory wider gait and compensatory trunk movements by the amputee during walking.